Polyimide is one of engineering plastics which, in recent years, has drawn attention especially in the electric and electronic, automobile, space, aircraft and the like industries due to its excellent heat resistance and mechanical characteristics. Thus, polyimide is expected to be in high demand. Since most polyimides are sparingly soluble or insoluble in organic solvents, a solution of a polyimide precursor which is soluble in an organic solvent has been used in various fields. As a polyimide precursor solution, a solution containing a poly(amic acid) represented by the following general formula is known. ##STR1## wherein R.sub.1 and R.sub.2 : aromatic residues
This poly(amic acid) solution is produced by reacting an aromatic diamine with an aromatic tetracarboxylic dianhydride in a solvent. Various solutions have been proposed. For example, the use of an aprotic polar solvent is described in JP-B-36-10999, JP-A-62-275165, JP-A-64-5057, JP-B-2-38149 and JP-B-2-38150, JP-A-1-299871, JP-A-58-12292, JP-B-1-34454, JP-A-58-185624, Journal of Polymer Science, Macromolecular Reviews, vol. 11, p. 199 (1976), U.S. Pat. No. 4,238,528, JP-B-3-4588, JP-B-7-3024, JP-A-7-41556, JP-A-7-62095, JP-A-7-133349, JP-A-7-149896, JP-A-6-207014, JP-B-7-17870, JP-B-7-17871, and IBM Technical Disclosure Bulletin vol. 20, No. 6, p. 2041 (1977) (the term "JP-A" as used herein means an "unexamined published Japanese patent application" and the term "JP-B" as used herein means an "examined Japanese patent publication"). Furthermore, the use of a mixed solvent selected from a water-soluble ether compound, a water-soluble alcohol compound, a water-soluble ketone compound and water is disclosed in JP-A-6-1915.
As a polyimide precursor which is a solute in the polyimide precursor solution, a variety of polymers other than the poly(amic acid) are known. For example, a poly(amic acid) ester represented by the following general formula ##STR2## wherein R.sub.3 : an aromatic residue
R.sub.4 : an aliphatic residue PA1 Me: a methyl group
is disclosed in Macromolecules vol. 22, p. 4477 (1989) and Polyimides and Other High Temperature Polymers, p. 45 (1991). Macromolecules, vol. 24, p. 3475 (1991) discloses a poly(amic acid) trimethylsilyl ester represented by the following general formula. ##STR3## wherein R.sub.5 and R.sub.6 : an aromatic residue
Journal of Polymer Science Part B, vol. 8, p. 29 (1970), Journal of Polymer Science Part B, vol. 8, p. 559 (1970), Journal of Chemical Society of Japan, vol. 1972, p. 1992, Journal of Polymer Science Polymer Chemistry Edition, vol. 13, p. 365 (1975) disclose a poly(amic acid) bis(diethylamide) represented by the following formula. ##STR4## wherein Et: an ethyl group.
The above-mentioned precursors are all polymers having a high degree of polymerization.
When polyimide coatings are obtained from solutions containing these polymers as a solute, generally, the polymer solutions are coated on a substrate such as copper, glass or the like, the solvent is removed, and imidization is conducted to form polyimide coatings.
However, when a polymer solution having such a high degree of polymerization is coated on a substrate, there is a problem in that the solid content must be reduced to provide a coating solution having a suitable viscosity. Furthermore, when the solid content is increased to raise productivity, the viscosity of the solution is increased, which makes coating very difficult. Still further, even if coating is possible, a coating or a film having excellent mechanical and thermal characteristics cannot be obtained. Furthermore, a polymer solution can hardly withstand long-term storage, and it is quite difficult to store a polymer for a long period of time while maintaining the polymerization degree of the polymer.